DC - Current Sensing Technology

The three most common ways to sense current are the resistive shunt, the current transformer, and the Hall effect current sensor.

Resistive Shunt

Photo 1. Sensors are available that are capable of measuring 0-15 kA DC to 200 kHz AC in a variety of packages.

The resistive shunt (so named because it is a shunt with respect to the voltmeter or system wiring) is simply a resistor placed in series with the load. According to Ohm's law, a voltage is developed across the shunt that is directly proportional to the current flowing through the load:

Most shunts are termed DC
shunts because of their inherent
added series inductance, which
limits the frequency response of
the device. The DC shunt
offers the lowest cost and most
accurate solution to low
current measurement
requirements where the
measured current is less than
~3 A. Additional advantages
and disadvantages of the DC resistive
shunt are summarized in Table 1.
If the resistive shunt is manufactured to
minimize it's inductance, it is termed an
AC shunt. AC shunts have the highest
operating frequency range of any current
sensing method, with 3 db points in the
several hundred megahertz range.

These shunts, however, are a great deal
more expensive than their DC
counterparts. The fundamental operating
characteristics of the AC shunt are the
same as the DC's (see table 2).

Current Transformer

The current transformer and the Hall
effect current sensor are based on the fact
that for a given flow, a proportional
magnetic field is produced in accordance
with Ampere's law. The current
transformer couples this magnetic field
into the secondary, providing a
proportional current output. The
operation of the device is identical to that
of any voltage step-up transformer. The
sensed, or aperture, current forms the
primary turn, while the large number of
turns wound on the magnetic core forms
the secondary. The turns ratio determines
the current output. For example, for a
single turn of 1000A, a secondary of 1000
turns would provide 1A of secondary
current. Two types of current transformers
are commercially available: high-volume,
low-cost, lower frequency devices and
lower volume, dramatically higher
frequency, higher cost research-grade
devices. The former is intended for low,
constant-frequency (60 Hz or 400 Hz, for
example) applications, while the research grade
devices generally focus on high frequency
(in the megahertz) RF and
pulsed applications. Table 3 summarizes
the characteristics of the low-cost type.

Table 3: AC Current Transformer Comparison

Advantages

Disadvantages

Low-cost method of measuring AC current (<100 A)

Measures AC current only

Provides voltage isolation

Produces AC insertion loss

Provides current output, ideal for noisy environments and easily converted to a voltage

Output is frequency dependent

Very reliable

Very large size and weight as measured currents increase

No external power requirements

Higher susceptibility to stray AC magnetic fields

Hall Effect Sensors

Hall effect current sensors (see Photo 1)
incorporate Hall generators, four-terminal
solid-state devices that output a voltage
proportional to the normal magnetic field
and the magnitude of the input control
current. Table 4 summarizes the pertinent
data. These detectors are either open loop
or closed loop; apart from their use of a
Hall generator, core and amp, the two
technologies are markedly different.

Open-Loop

Figure 1. The open-loop Hall effect current
sensorrelies on the magnetic core to concentrate
the magnetic field onto the Hall generator. The
signal is then amplified to a useful output.

Open-loop Hall effect current sensing is
the easier to understand. The hall
generator is mounted in the air gap of a
magnetic core. A current-carrying
conductor placed through the aperture of
the core produces a magnetic field
proportionate to the current. The core
concentrates the magnetic field, which is
measured by the Hall generator. The signal
from the Hall generator is low and is
therefore amplified to a useful level, which
becomes the output of the sensor. (see
Figure 1).
To understand open-loop Hall effect
current sensing, some fundamentals of the
Hall generator must be reviewed. Recall
that the Hall generator is a four-terminal
solid-state device most commonly made of
a thin film of silicon, germanium, indium
arsenide, indium antimonide, or gallium
arsenide. Two leads provide the voltage
output; the other two require a voltage or
current source input. The voltage output is
a differential voltage between the two
leads that is dependent on the normal
magnetic field and the control current
flowing through the input leads. Certain
basic characteristics of the Hall generator
vary over temperature, such as I/O
resistance, misalignment voltage (offset
voltage), and sensitivity. When designing
with Hall generators, a constant current
source is therefore used to power the device, allowing the output to be a
function of the incident magnetic field and
not dependent on the temperature-varying
input resistance. A constant current source
is also added to help correct the change in
sensitivity over temperature. Typically, the
sensitivity of an indium arsenide or gallium
arsenide Hall generator drifts approximately
-0.05%/ºC over temperature. This drift can
be minimized by injecting a positive
0.05%/ºC coefficient in the control current,
which significantly reduces the drift in
sensitivity over temperature. (For clarity,
the temperature-compensated constant current
source is not shown in Figure 1).
The open-loop method thus contains four
building blocks: the Hall generator, the
magnetic core, the amplifier, and the
temperature-compensated constant-current
source. The linearity of the open-loop
sensor is subject to the linearity of the core
and the Hall generator. The offset drift over
temperature is determined by the offset drift
of the Hall generator and the amplifier, and
the gain of the amplifier.

Closed-Loop

Figure 2. The closed-loop current sensor adds
a push-pull amplifier that drives a nulling coil.
This results in the sensors always operating at or
near zero magnetic flux, eliminating dependence on
the linearity of the core and hall generator.

Closed-loop Hall effect current sensors
have five basic building blocks: the Hall
generator, the magnetic core, the amplifier,
a driver circuit, and a coil wound in series
opposition around the magnetic core. The
term closed-loop is used because the magnetic field generated by the current carrying
conductor is nulled within the
magnetic circuit of the core, thus closing
the magnetic loop. This technique allows
great improvements in sensor performance.
The Hall generator senses the magnetic
field generated by the conductor under
measure and concentrated by the magnetic
core. (see Figure 2).
The Hall generator
output voltage is amplified by a very large
gain amplifier, essentially operating at an
open-loop (no feedback) gain. The
amplifier output is fed into a push-pull
driver stage that drives the coil wound in
series opposition on the magnetic core.
This produces a magnetic field that is equal
but opposite to the magnitude of the
aperture current, thus driving the core to
near zero flux. The output of the sensor is
one lead of the coil, and connecting it to
ground through a sense resistor completes
the circuit. Operating the core at near zero
flux eliminates the dependence on the
linearity of the core and Hall generator
and also effectively eliminates the
temperature dependence of the Hall
generator's sensitivity. The Hall generator
will still drift in sensitivity over
temperature, but this drift times an
effectively zero field equals zero. The
generator therefore does not need a
temperature-compensated constant-current
source.

Table 4: Hall Effect Current Sensing Comparison

Advantages

Disadvantages

Measures DC and AC currents

Outputs signal for zero current flow (has offset)

Lowest cost method of measuring larger AC and DC currents (>500 A)

Requires external power supply

Provides electrical isolation

Difficult to understand

Very reliable

Technical considerations required for over temperature performance, overcurrent and power supply variations

The output of the closed-loop sensor is
therefore proportionate to the aperture
current and the number of turns of the
coil. A sensor with a 1000-turn coil will
provide an output of 1mA/A. This output
current is then converted to a voltage by
connecting a resistor to the output of the
sensor and ground. The output can be
scaled by selecting different resistor values
within specified limits.

Open and closed-loop compared

Each of these two Hall effect technologies
offers technical and economical advantages
(see Table 5). The closed-loop sensor has
superior linearity, low temperature drift,
fast response time, and a wide frequency
range. Open-loop technology offers excellent
performance with respect to price and is
preferable for battery-operated applications
where power consumption, size, and
weight are dominant concerns. Two
points identified in Table 5 require further
explanation. As previously noted, the
open-loop sensor is the device of choice
for low power consumption designs because
it consumes the same amount of power
regardless of aperture current. The closedloop
sensor requires more current as the
aperture current increases because more
current is required to null the flux.
The second point is that open-loop sensors
can operate at excessive overcurrents
indefinitely and suffer no damage. The
closed-loop sensor cannot because the
larger the aperture current, the larger the
push-pull driver current and the higher the
heat dissipation in the coil and the sense
resistor. These components all have power
limits, a characteristic that limits the sensor's
overcurrent measuring capability. When
considering an overcurrent condition using
a closed-loop sensor, the designer must
consider both the peak aperture current
and the duty cycle of that current. If the
highest performance is required by the system
design, the closed-loop sensor is the sensor
of choice. However, this improved
performance comes at a cost in weight and
dollars (see Figures 3 and 4), both of which increase with the closed loop
sensor's current rating.
(Note that the prices are from the
manufacturers' catalog and are used only to
indicate the relative differences in cost as
the current rating of the sensor increases.
This is due to the added turns and current
required to null the larger magnetic fields.
As the currents increase, a single drive
stage is no longer adequate and larger
compliance voltages are required. This
results in more complex circuitry, more
weight and additional cost).

Applications

Hall effect current sensors are used in
power supplies, motor drives, and general
load applications.

Power Supplies

Used in virtually all electronic equipment
in one form or another, power supply
applications range from lasers to computers
to nuclear power plants. A power supply
may use current sensing to shut down
during an overcurrent condition and
protect internal components from damage
or for personal safety. The sensor can also
be used as a feedback element to regulate
the current, as in electroplating, arc
welders and battery chargers.

Motor Drives

Trains, factories, elevators, and air
handlers all rely on motor drives, where
motor torque is proportional to the
motor's electrical current. Monitoring the
current provides load information. If the
motor becomes unloaded, as in the case of
a broken tool bit or cavitating pump, the
current decreases and can be used to signal
an operator to replace the bit or reprime
the pump. Motor drives not only allow
greater control of the motor but also
dramatically increase efficiency, which
saves money when it comes time to pay
the electric bill.

General load applications

Figure 3. Due to the added output stages and
increased number of windings required to null the
larger magnetic fields, the closed-loop current sensor
dramatically increases in weight as the measured
current increases.

These are a diverse group. Lighting on
airport runways and tall structures (cellular
towers, for example) is required for safety.
Measuring the current to those lights
provides feedback on their condition. If the circuit opens (no current), an
automated signal could be generated to a
technician to indicate the need for a bulb
change. In plastic injection molding
machines, the plastic is preheated to a
given temperature to insure a proper
mold. If a heater element burns out, the
plastic will not be at the appropriate
temperature, causing a yeild problem. A
current sensor monitoring the heater
current can signal an operator if the
element burns out.

Table 5: Open-Loop vs. Closed-Loop Hall Effect Current Sensing

Advantages of Open-Loop

Lowest cost in higher current ranges (>100 A)
Low

Constant power consumption regardless of sensed current
Smallest size

Lowest weight in higher current ranges (>100 A)

No damage from excessive overcurrents (>10x rating)

Advantages of Closed-Loop

Highest accuracy at ambient and over temperature

Provides a current output, ideal for noisy environments
and easily converted to a voltage

Higher frequency range (>150 kHz)

No magnetic hysteresis of offset

Trends in current sensing

Figure 4. Similar to the case in Figure 3, the cost
of the closed-loop current sensor increases with the
measured current.

As system requirements increase, with
voltage slew rates >5000 V/ms and current
slew rates >200 A/ms, demands on the
sensor also increase. Researchers are
investigating not only improvements in
Hall effect sensing but also alternative
technologies such as magnetorestrictive,
magnetoresistive, magneto-optic,
magnetodiode, and magnetotransistor
sensors. There is movement toward selftest
features, more durable packaging, and
smaller footprints. Split-aperture sensors,
or configurations where the core is split in
two, greatly facilitate both initial and field
service installation. Development of a 4-20
mA output current sensor allows simple
system interface to a PLC. Higher noise
immunity is of particular interest because
as the electronics become more remote
from the sensor installation, frequency and
system noise increase.